Basic Corr Chapter 2-Jan 2003-New Logo

8/11/2019 Basic Corr Chapter 2-Jan 2003-New Logo

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Definition of Terms

Matter

Element

Compound

MixtureAtom

Molecule

Ion

Electrolyte


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Matter

Matter is any substance that has

weight and occupies space.


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Element

An element is a substance that cannot

be broken down by chemical reactions

Elements are the basic building blocks

of all matter

ExamplesIron, Oxygen, Hydrogen,

Gold


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Periodic Table

PERIODIC

TABLE


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Common Elements

Iron, Fe

Copper, Cu

Sodium, Na

Chlorine, ClOxygen, O


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Compound

A compound is a combination of two

or more elements

A compound is a pure substance

A compound has a fixed compositionExamples of compoundscarbon

dioxide, ferric oxide, salt, water


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Mixture

A mixture is a combination of elements and/or

compounds held together by physical rather

than chemical forces

Mixtures do not have fixed compositions

Examples of mixturesair, soil, most rocks


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Atom

An atom is the smallest particle of an

element

An atom consists of a central nucleus that

has a positive charge surrounded by

electrons that have an equivalent negative

charge

Atoms are electrically neutral


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ELECTRONS

NUCLEUS

ORBIT

Bohr Model of an Atom


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Molecule

A molecule is the smallest particle of an

element or compound that retains all of the

properties of that compound or element

The smallest particle of an element

hydrogen gas is H2 ;oxygen gas is O2

The smallest particle of a compoundwater

is H2

O; ferric oxide is Fe2

O3


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Oxygen

Atom

Hydrogen Atoms

Water Molecule


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Ion

An ion is a charged atom or molecule

An atom or molecule with a netnegative charge is a negative ion,also called an anion

An atom or molecule with a netpositive charge is a positive ion, alsocalled a cation

(Atoms or molecules become ions by

transferring electrically charged

particles.)


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Electrolyte

An electrolyte is a liquid solution

containing ions

An electrolyte can conduct an electricalcurrent by the movement of ions


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Bonding Forces

Van der Waalsinert gases

Ionicdissimilar charged ions such as

NaCl

Covalentshared valence electrons

such as O2

Metallicvalence shell electrons


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Corrosion reactions aresometimes called

Electrochemical Reactions

Oxidation/Reduction Reactions

Redox Reactions

Corrosion Cells


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Oxidation

Oxidation occurs whenelectrons are lost from anatom, compound, or ion

Oxidation reactions increase

the positive charge of theatom, compound or ion

Oxidation occurs at theanode in an electrochemicalreaction

The electrons remain in themetal and may move freelywithin the metal


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Reduction

Reduction occurs when electronsare added to an atom,compound, or ion

Reduction reactions increase thenegative charge of the atom,

compound or ionReduction occurs at the cathodein an electrochemical reaction

The electrons are supplied fromwithin the metal and move to the

cathode from anodic sites


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Anodic Reactions

Electrons are given up inanodic reactions

Positive charge increases

Negative charge decreases

Anodic reactions are calledoxidation

The electrons remain in themetal and may move freely

within the metal


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Anodic Reactions

Generic: MO Mn+ + ne

Iron: Feo F e++ + 2e

Feo F e+++ + 3e

Fe++ F e+++ + eAluminum: Alo Al +++ + 3e

All anodic reactions make the reacting

metal more positive and produceelectrons


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Cathodic Reactions

Electrons are gained in cathodicreactions

Negative charge increases

Positive charge decreases

Cathodic reactions are calledreduction

The electrons are supplied from

within the metal and may move to

the cathode from other sites


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Cathodic Reactions

Generic: R++ e RO

Hydrogen Reduction:

H+ + e H O

2 H O H 2

Oxygen Reduction:

2 H2O + O2+ 4 e 4(OH)

O2+ 4 H++ 4 e 2 H2O

All cathodic reactions make the

reacting material more negative(less positive) and consume

electrons


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Electron vs. Conventional Current Flow


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Which direction is conventional flow in this cell?

Which direction is conventional flow


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Which direction is conventional flow

in this cell?


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The Corrosion Cell

Metallic corrosion is an electrochemical

reaction which involves:

a transfer of electrons

oxidation - loss of electrons (corrosion) reduction - gain of electrons

(protection)

migration of ions


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Corrosion Cell Drawing


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4 Requirements for a Corrosion Cell

Anode

Areas that give up electrons

Cathode

Areas that receive or consume

electronsMetallic Path

Areas that transport electrons

Electrolyte

Areas that transport ions

Anode


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Anode

Which direction

is conventional

flow in this cell?

Which direction

is electron flowin this cell?


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We now know how corrosion happens

it is electrochemical

The electropart is the electrons

The chemicalpart is the ions

Next we need to know what causes

th is process to happen.


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Review

Most corrosion of metals is

electrochemical

There are 4 requirements for

electrochemical corrosion to

occur

Corrosion can be controlled by

removing or slowing any of the

4 requirements


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What causes electrons to flowin a corrosion cell?

Corrosion cells are created when there

is an imbalance or non-equilibrium

energy balance, causing a movement of

energy from high levels to lower levels.

Thermodynamics


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Potential or emf (Electromotive Force)

Electrical (electrochemical) potential is a

measurement of energy level

More negative potentials indicate higher

energy levelsPotential is measured in volts


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Potential Measurement

Detection of anodes and cathodes can

be accomplished in many cases with a

voltmeter and reference electrode.

A corrosion potential is made bycomparing a metal sites voltage to a

reference electrodes voltage.


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Corrosion Potential


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Reference Electrodes

Reproducible Potential

Hydrogen electrode = arbitrary zero

CalomelMercuryPotassium Chloride

Saturated, 1 Normal, 0.1 Normal

Silver/Silver Chloride

Wet typePotassium Chloride

Saturated, 1 Normal

Dry TypeSeawater

Copper/Copper SulfateSaturated


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0.7755 V

Comparison of Potentials MeasuredUsing Different Reference Electrodes

Hydrogen Reference Electrode

0.000 V

0.850 V

Saturated Calomel Reference Electrode

+0.2415 V

Copper / Copper Sulfate

+0.3160 V

0.534 V

Metal Surface


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Copper- Copper SulfateReference Electrode

Copper Rod

Saturated Copper

Sulfate Solution

Undissolved Copper

Sulfate CrystalsPorousPlug

VisualWindow


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Reference Electrode Potentials

ReferenceElectrode

Hydrogen

Calomel (Saturated)

Calomel (Normal)

Calomel (0.1) Normal

Silver/Silver Chloride (Saturated)

Silver / Silver Chloride (Normal)

Silver /Silver Chloride (Dry inSeawater)

Copper / Copper Sulfate

Potential(Volts)

0.000

+ 0.2415

+ 0.2800

+ 0.3337

+ 0.2250

+ 0.2222

+ 0.2500

+ 0.3160


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Corrosion Potential

The voltage difference between a corroding

surface and a reference electrode is a

potential


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Galvanic Series

Listing of metals in order of potential

Different for different environments

Different than the emf series

Does not give corrosion rates


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The Nernst Equation

Used to calculate the change in potential

that results from changes in the

electrolyte.

lnnF

RTEE

reaction products

reactants


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Pourbaix Diagrams

Predicts stability of substances

Considers potential and pH

Predicts possibility of corrosion

Predicts corrosion productcomposition

Evaluates effect of changes in pH or

potential


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0.0

1.0

2.0

-1.0

-2.0

0.0

1.0

2.0

-1.0

-2.0

-2 0 2 4 6 8 10 12 14 16

-2 0 2 4 6 8 10 12 14 16

POTENTIAL,

S.H.E.

(volt)

pH

Fe

Fe++

Fe+++

Fe2 O3

Fe3 O4a

b

HFe O2-

Pourbaix Diagrams


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Kinetics

Defines corrosion rate

Usually predicts corrosion current density

Corrosion rate is proportional to current

density

Evans ( E vs log i ) diagrams


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Open Circuit Potentials

More Positive

(+)

x Open circuit potential

of the cathode

E

More Negative

(-)

x

Log i

Open circuit potential

of the anode


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Anode Polarization

More Positive(+)

E

Anode Polarization

More Negative()

Log i

x

x


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Cathode Polarization

More Positive

(+)Cathode Polarization

E

More Negative

(-)Log i

x

x


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Combined Polarization Complete Cell

More Positive

(+)x

E

Corrosion

CurrentxMore Negative

(-)

Log i


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Area Effects

Reaction intensity is dependent on

current density

Current densitycurrent per unit

areasuch as mA/cm2

Current density affects polarization


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Area Effects - Smaller Cathode

More Positive

(+) x

E

New

Original

Corrosion

Currentx CorrosionMore Negative Current

(

)

Log i


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More Positive(+)

x

E

xMore Negative

(

)Log i

Area Effects - Smaller Anode

Original

Corrosion

Current

New

Corrosion

Current


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Types of Electrochemical Cells

Any difference in potential can result in anelectrochemical corrosion cell Difference in MetalGalvanic Corrosion Cells

Difference in ConcentrationConcentration Cells Difference in Surface Activity - Active/Passive Cells

Difference in TemperatureThermogalvanic Cells


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Reversal of Potential Zinc-Iron

More NegativePotential of Zinc

PotentialPotential of Iron

More

Positive100 F

Temperature140 F

180 F


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Passivity

Formation of passive films of reactionproducts:

May occur naturally Stainless Steels

Nickel Alloys Titanium

May be formed chemically orelectrochemically

Oxidizing solutions

Applied current


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Transpassive

Passive

Active

Corrosion Rate

OxidizingPower

Passivity


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Summary

Thermodynamics

Kinetics

Nernst Equation

Faradays Equation

Polarization

Passivity


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Basic Corr Chapter 2-Jan 2003-New Logo

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